Central nervous system impairment induced by scopolamine in healthy volunteers has been advanced as a model of Alzheimer's disease to assist in choosing doses of new cholinergic agents to be used in clinical efficacy trials in dementia patients. This study is to improve the utility of the model by extending it to healthy elderly volunteers to incorporate age-associated physiologic changes that can alter the pharmacodynamics and Pharmacodynamics of drugs under investigation and to examine the time course of drug concentrations and effects.